Optical color image scanners use photodetectors to convert an image into an electronic signal suitable for use in copying, electronic display, storing or printing. Photosensor elements provide a voltage or current signal that varies with the light intensity impinging onto the photosensor element. Photosensors are not inherently color specific. Typically, a color image scanner uses one of two methods to measure specific color bands within broad spectrum incident light. In a first method, light passes through a color filter before impinging onto a photosensor element. In a second method, color separators (typically prism based) are used to separate broad spectrum light into physically separated bands of color. The first method, using filters, is of primary interest in the present application.
Video cameras (cameorders), digital cameras and similar devices often use a two dimensional matrix photosensor array in which there are many rows and columns of photosensor elements, each element being covered by an individual color filter. Document scanners and slide scanners often use a photosensor array in which there are three or four rows of photosensor elements, each row receiving light having a narrow bandwidth. Both types of photosensor arrays are of interest to the invention.
One purpose of a color image scanner is to provide a digital representation of an image that can then be displayed or printed, with the displayed or printed image appearing identical to the original to a human observer. Typically, a color scanner transforms the light from an individual picture element into three numbers, each number representing a dimensional value in a three dimensional color space. Displays or printers then transform the values into a different color space appropriate for display phosphors or printing pigments or dyes. See, for example, K. Douglas Gennetten and Michael J. Steinle, "Designing a Scanner with Color Vision," Hewlett-Packard Journal, August, 1993, pp 52-58. See also, U.S. Pat. No. 5,285,271, Digital Color Matrixing Circuit to K. Douglas Gennetten. Numerical values resulting from the photosensor signals must also be transformed to compensate for non-ideal filters and non-ideal illumination in the scanning process. Because of limited digitization, arithmetic roundoff, arithmetic truncation, and noise problems, arithmetic compensation is limited. Therefore, it is very important for the filters to be as close to ideal as possible. Ideally, color measurement systems match the human visual system. In particular, for the color red, human sensitivity peaks at wavelengths of about 660 nanometers and falls to nearly zero at about 700 nanometers.
Transmissive color filters are typically thin film organic dyes deposited directly onto the photosensor array, which in turn is typically fabricated on silicon in an integrated circuit process. While thin film organic dyes are well suited for an integrated circuit process, they are not the best technology for precise shaping of the filter characteristic. In general, organic dyes cannot provide a red filter having an ideal filter characteristic. In addition, organic dye filters typically transmit infrared wavelengths in addition to a narrow band in the visible wavelengths. Therefore, when using organic dye filters directly deposited onto photosensor arrays, additional filtering is needed elsewhere in the system. Lens coatings for suppressing infrared wavelengths are known. However, coating a curved lens surface is relatively expensive. Inserting additional filters in the light path is known. However, this adds an additional part. There is a need for a compensation filter having ease of manufacturing and low cost.